1. Field of the Invention
The invention relates to a liquid crystal display device, and more particularly to technologies for enhancing the structure of the peripheral area of the substrate of a liquid crystal display.
2. Description of the Related Art
In conventional liquid crystal display devices, the sealing of a liquid crystal layer between two substrates, and the application of voltage between the electrodes formed in pairs on the back side of the liquid crystal layer change the orientation of the liquid crystals inside the layer, making it possible to display various images. FIG. 7 depicts an example of an enlarged cross-sectional view of the substrate edge area of a conventional liquid crystal display devices, and FIG. 8 is a perspective view from above showing the overall configuration.
As shown therein, both the top and bottom substrates 1, 2 are constituted by clear glass plates, and transparent electrodes 3 and 4 are formed in specified patterns on the opposing internal surfaces of substrates 1 and 2. Liquid crystal layer 5 is sealed between top substrate 1 and bottom substrate 2 by means of sealant 6.
Bottom substrate 2 extends farther out to one side than top substrate 1, and electrode pads 4a, which are conductively connected to clear electrode 4, constitute external electrodes are formed on this surface. Insertion pins 7, installed such that they clamp bottom substrate 2, contact these electrode pads 4a and connect to an external wiring substrate, or the like. Insertion pins 7 with legs are attached to substrate 2 by means of elastic jaws 7a and 7b, such that they clamp onto bottom substrate 2; and these installation areas are secured with molding material 8.
Multiple electrode pads 4a are positioned at a specified minimum interval on the edge area of bottom substrate 2, and are connected to transparent electrodes 4 formed on the interior surface of bottom substrate 2. On the other hand, as shown in FIG. 8, multiple electrode pads 3a are also formed on the edge area of bottom substrate 2, and are connected via conductive connection area 10 to transparent electrodes 3 formed on the interior surface of top substrate 1. The above device is, for example, a segment-type liquid crystal display device, and transparent electrodes 3 and 4 are used as common electrodes and segment electrodes, respectively.
Such conventional devices have numerous drawbacks problems and has a disadvantage for example in the above-mentioned conventional liquid crystal display device, it is necessary to fill and coat the gap between top substrate 1 and bottom substrate 2 with resin molding material 8 in order to secure the area where electrode pads 3a and 4a and insertion pins 7 with legs are conductively connected, and to prevent electrolytic corrosion of electrode pads 4a, in the electrode lead-out area where bottom substrate 2 extends outward beyond the edge area of top surface 1. This kind of coating is also used in making a conductive connection for the pad areas of a circuit board via bonding wires, and in making a conductive connection using a conductive adhesive in the edge area of a flexible substrate and heat seal, in addition to the conductive connection that is made via insertion pins 7 with legs as described above.
The reasons for using a resin mold for coating include the following: to electrically coat the connections of the various components connected to the above-mentioned insertion pins 7 with legs, bonding wires, flexible substrates, heat seal, etc. (conductive connection area protection mold); to secure flexible substrates and the heat seal, etc. and to increase their rigidity (reinforcement mold); to prevent electrolytic corrosion caused by the potential difference between electrode pads resulting from the adhesion of a liquid such as water to two or more adjacent electrode pads 3a and 4a (anti-corrosion mold); and to prevent corrosion and erosion of other structural areas of a liquid crystal panel (durability and corrosion resistance enhancement mold).
As explained above, this kind of mold is often applied to the conductive connection area where electrode pads 3a and 4a are formed. However, the type of reinforcement and durability mold described above is necessarily applied to areas other than conductive connection areas. Therefore, resin coating using a mold agent is sometimes applied to the entire perimeter of a liquid crystal panel.
When such a mold, or coating, is applied, the hardening and shrinking of the mold resin usually pulls the edges of the top and bottom substrates closer, deforming them inward. As a reaction to this phenomenon, the space between the top and bottom substrates becomes wider toward the middle of sealing material 6, causing negative pressure inside, and as a result, low-temperature air bubbles and shock air bubbles tend to occur inside the liquid crystal layer.
Furthermore, the stress accompanying the resin coating changes the cell thickness of the liquid crystal panel, and tends to produce coloring and color irregularity. This cell thickness change tends to be particularly large near the seal area, and it may become necessary to apply a black mask to the perimeter of the liquid crystal panel in order to avoid display quality degradation in this area. Because the formation of this mask reduces the liquid crystal display area, it increases the overall cost of manufacturing a liquid crystal display. The above-mentioned coloring and color irregularity become particularly serious problems in large-size liquid crystal panels since cell thickness may vary widely within the panel surface.
The cause of the stress that brings about the above-mentioned substrate deformation is not limited to shrinkage-induced stress, and may include such factors as mechanical contacts made during liquid crystal panel manufacturing or during the incorporation of the liquid crystal panel into various instruments, and shock or vibration applied to the various instruments into which the liquid crystal panel is incorporated.
Therefore, it is the object of the present invention to solve the above-mentioned problems and provide a new structure for a liquid crystal display device, that can prevent substrate deformation by dispersing the stress applied to the substrates.
It is another object of the present invention to improve the display quality of liquid crystal display devices by reducing dimensional errors in the liquid crystal cell structure caused by external stress.
It is a further object of the present invention to improve product yield by preventing quality degradation of liquid crystal display devices due to mold formation.
It is a still further object of the present invention to achieve a method that does not require any special design change or addition of manufacturing steps in order to prevent substrate deformation caused by mold formation.
As a means of solving the above-mentioned problems, the liquid crystal display device of the invention is provided with two substrates at least one of which is transparent, a liquid crystal contained between the substrates, a seal area formed between the substrates to surround the liquid crystal layer, and support columns for connecting the substrates on the outside of the seal area. The liquid crystal layer can be based on any display principle, and the seal area and the support columns are not limited to any particular materials. Furthermore, any number of support columns can be used, and they can be formed in any shape.
According to this means, the formation of the support columns on the outside of the seal area reduces substrate deformation that may be caused by various elements that use the seal area as the function. Therefore, it is possible to reduce the occurrence of air bubbles inside the liquid crystal layer, as well as undesirable coloring of the display area, and color irregularity.
When a mold material for covering the edge areas of the substrates is provided, the support columns resist the compression force of the mold material, thus reducing the stress applied to the substrates and providing the effect of suppressing substrate deformation. The mold materials include a type for fastening connection components, e.g., connection pins and connection wires, that are conductively connected to the external pins used for applying an electrical field to the liquid crystal layer; a type for reinforcing the flexible substrate that is conductively connected to the external pins used for applying an electrical field to said liquid crystal layer; and a type for preventing electrolytic corrosion of said external pins.
In these cases, the support columns should preferably be formed at the edges of the substrates. The support columns formed at the edges of substrates offer the greatest resistance to the stress applied to the edges of the substrates.
Furthermore, the support columns should preferably be formed using the conductive paste used for making a conductive connection among the wires inside the device. When the support columns are formed using the conductive paste, the support columns can be formed simultaneously with the conductive connection areas (in particular, the top and bottom connection areas for connecting the conductive elements formed along the substrates that sandwich the liquid crystal cells) inside the liquid crystal panel, or the support columns can be used as the conductive connection area. In this way, manufacturing costs can be reduced without any changes in the number of processes or the process contents.
Additionally, the conductive paste should preferably be a conductive ink containing carbon. When a conductive ink containing carbon is used, the support column can easily and precisely be formed using a printing method.
Furthermore, the support columns should preferably be formed using a material containing a phenol resin as the main ingredient, in which case their adhesion to the substrates is relatively weak and they process an appropriate degree of flexibility. This facilitates the cutting of the substrates which is performed during the manufacture of the liquid crystal display device, and avoids the risk of cracking the substrates when they are cut at the areas where the support columns have been formed.
Additionally, the support columns should preferably be formed using a material processing weaker adhesion strength than the material comprising the seal area. This facilitates the cutting of the substrates which is performed during the manufacture of the liquid crystal display device, and avoids the risk of cracking the substrates when they are cut at the areas where the support columns have been formed.
Furthermore, the support columns should preferably be formed using a material processing the same hardening characteristics as the material comprising the seal area. When the support columns are formed using a material that possesses the same hardening characteristics as the material comprising the seal area, the support columns can be hardened simultaneously with the seal area during the manufacture of the liquid crystal display device. Therefore, unlike in a case in which the support columns and the seal area are separately hardened, the substrate positioning accuracy used for hardening the seal area can also be used for the support columns, thus avoiding increases in the number of manufacturing steps and the process time.
Additionally, according to the manufacturing method of the liquid crystal display device, a seal area is formed between two substrates at least one of which is transparent, to surround the area that houses the liquid crystal layer, and after the support columns for connecting the substrates on the outside of the seal area are formed, the edges of said substrates are covered by a mold material. In this manufacturing method, the above-mentioned effects can be easily obtained simply by providing the support columns.
Note that the seal area and the support columns should preferably be formed using materials possessing the same hardening characteristics, and the seal area and the support columns should preferably be hardened at the same time. The seal area and the support columns can be hardened simultaneously if they are formed using materials possessing the same hardening characteristics, e.g., thermohardening or photo-curing characteristics, thus eliminating the need for a new hardening process.
Furthermore, the conductive connection areas that connect electrodes formed on the substrate with external pins should preferably be formed simultaneously with the support columns using the same material. The support columns and the conductive connection areas can be positioned and formed using a single process if they are formed using the same material, thus eliminating the need for a new manufacturing process for positioning and forming the support columns, and the above-mentioned effects can be obtained by merely changing the formation pattern of the conductive connection areas.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.